1. Field of the Invention
The present invention relates broadly to the art of electroless deposition of metallic films onto non-metallic substrates. More particularly the invention relates to improving the method of preparation and the plating efficiency of an electroless gold plating bath for the deposition of uniform gold films on glass.
2. Description of the Prior Art
In the art of depositing noble metal coatings onto metallic surfaces, U.S. Pat. No. 2,969,295 discloses adding potassium gold cyanide to a sodium bicarbonate solution, heating the solution to facilitate more rapid plating, and immersing metallic bodies. Chemical plating occurs because of the tendency for metals lower in the electromotive force series to displace noble metal ions from the plating solution.
In the art of depositing noble metal coatings onto non-metallic surfaces, U.S. Pat. No. 3,300,328 to Luce discloses an aqueous electroless gold plating bath comprising a gold compound, an ammonium or alkali metal sulphite or meta-bisulphite complexing agent, and a hydrazine or hydroxylamine reducing agent. Gold films are deposited in about 40 minutes at elevated temperatures.
A more rapid method of depositing gold films onto non-metallic substrates is described by Levy in U.S. Pat. No. 3,515,571. A preferably neutral gold solution is prepared by dissolving in water a gold salt such as gold chloride, then complexing the free gold ions in excess of 10.sup.-16 gram ions per liter by suitable coordinating ligands such as alkali metal carbonates, alkali metal hydroxides, ammonia and amines. Levy emphasizes the importance of mixing the ingredients in the following manner. First, gold salt is dissolved in water and neutralized with sodium hydroxide. Next, the ligand is dissolved in water and neutralized with an acid. Finally, the gold salt solution and ligand solution are mixed together to form a final neutral gold plating solution. Gold films may be deposited on non-metallic substrates in about 1 minute at ambient temperatures by contacting a receptive surface with the above final gold solution and a second solution of a hydrazine reducing agent. Levy suggests the use of the resultant gold coated articles as conductors, electrodes, and mirrors.
U.S. Pat. No. 3,484,263 to Kushihashi et al. discloses a method for forming a homogeneous semi-transparent gold coating on glass. The method involves contacting a sensitized glass surface with an alkaline aqueous solution of a gold salt, a reducing agent and an alkali carbonate to promote reduction at a temperature not to exceed 10.degree. C. After about 0.5 to 5 minutes contact, the contacting interface is subjected to radiation of 2500-5000 Angstroms to reduce the gold salt to a gold coating with a thickness of 150-500 Angstroms.
In U.S. Pat. No. 589,234, filed on June 23, 1975, now U.S. Pat. No. 4,005,229, Miller et al. disclose an improved method for the rapid deposition of uniform transparent gold films on non-metallic substrates such as glass. The method involves contacting an activated surface with an aqueous solution of a gold salt and a complexing agent in the presence of a hydrazine or hydroxylamine reducing agent. The improvement concerns aging the gold solution and contacting the surface in the presence of divalent mercury, cadmium or lead ions in order to enhance the uniformity of the gold films, an extremely important consideration if the coated articles are to be used in architectural applications.
The present invention provides a fast, simple, commercially practicable method of depositing gold films on non-metallic substrates such as glass and further provides alternative methods for improving the uniformity of such films for aesthetically pleasing coated articles having excellent solar energy control properties.